Stiffener with core and shell

ABSTRACT

A stiffener is disclosed including a core and a shell which surrounds the core. The shell is formed from a fibre material, and the core includes first and second battens arranged side by side, and a foam spacer between the battens. The stiffener extends in a lengthwise direction, and the battens and the foam spacer have respective lengths which extend in the lengthwise direction of the stiffener. The core is assembled with the spacer between the battens, then surrounded with the shell. The stiffener may be a stringer for an aircraft wing.

CROSS RELATED APPLICATION

This application claims priority to United Kingdom Patent Application GB2115992.6 filed Nov. 8, 2021, the entire contents of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a stiffener, and a panel assemblyincorporating such a stiffener.

BACKGROUND OF THE INVENTION

Composite stiffeners have a variety of different forms. One well-knownform is a so-called “blade” stringer. Blade stringers have a relativelyhigh aspect ratio (depth/width) which can result in variousdisadvantages: for example the free edge of the blade may be prone todamage, and the blade may be prone to buckling.

Another form is an “omega” or “hat” stiffener. Hat stiffeners have arelatively low aspect ratio (depth/width) which can avoid thedisadvantages of blade stringers, but can also introduce differentproblems: for instance increased weight, and increased pitch between thestiffeners.

An example of a “hat” stiffener is disclosed in US2010129589. In oneexample, composite material is laid up over foam which is placed into aninside mold line tool. Then fuselage skin is placed or laid up onto thecomposite material, foam, and inside mold line tool. A problem with thismanufacturing process is that the stiffener must be assembled on theinside mold line tool, which can make the process complex and difficultto automate.

An aircraft panel assembly disclosed in WO2020/229501 comprises a panel,and a plurality of stiffeners on the panel. Each stiffener has anattachment part attached to the panel and a structural part spaced apartfrom the panel. A rib foot beam crosses the stiffeners at a series ofintersections. At each intersection the rib foot beam is located betweenthe panel and the structural part of a respective one of the stiffeners.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a stiffener comprising: a core;and a shell which surrounds the core, wherein the shell is formed from afibre material, and the core comprises: first and second battensarranged side by side; and a spacer between the battens, wherein thestiffener extends in a lengthwise direction, and the battens and thespacer have respective lengths which extend in the lengthwise directionof the stiffener.

Optionally the battens are spaced apart by a gap; the spacer is in thegap between the battens; the battens, the spacer and the gap haverespective widths; the widths of the battens decrease in the lengthwisedirection; and the widths of the spacer and the gap increase in thelengthwise direction.

Optionally the core has a width which does not change substantiallyalong the length of the stiffener.

Optionally the spacer is formed from a foam material.

Optionally the battens are formed from a fibre material, which isoptionally a fibre-reinforced composite material.

Optionally the fibre material of the shell is a fibre-reinforcedcomposite material.

Optionally the battens have a higher mass per unit volume than thespacer.

Optionally the stiffener further comprises a cap between the battens andthe shell.

Optionally the battens have a higher mass per unit volume than the cap,and/or the cap is formed from a foam material.

Optionally the core has no more than two battens.

Optionally the shell has a depth and a width transverse to the length ofthe stiffener, the battens are spaced apart across the width of theshell, and the depth of the shell is greater than the width of theshell.

Optionally the shell comprises: a foot; a crown opposite the foot; afirst side wall; and a second side wall opposite the first side wall.

Optionally each sidewall is longer than the foot, viewed in sectiontransverse to the lengthwise direction of the stiffener.

Optionally the first batten is adjacent to the first side wall, and thesecond batten is adjacent to the second side wall.

Optionally the first and second side walls are substantially parallelwith each other.

Optionally the shell has a substantially rectangular outer profile.

Optionally the battens are not in contact with each other at any pointalong their respective lengths.

Optionally each batten and each spacer extends continuously along a fulllength of the stiffener, or at least along a majority of a full lengthof the stiffener.

A second aspect of the present invention provides a stiffened panelassembly comprising: a panel; and a stiffener according to the firstaspect adhered to the panel.

Optionally the shell comprises: a foot; a crown opposite the foot; afirst side wall; and a second side wall opposite the foot, wherein thefoot of the shell is adhered to the panel.

The shell has a shell depth between the foot and the crown; and a shellwidth between the side walls. Optionally the shell depth is greater thanthe shell width.

Optionally each batten comprises an inner edge facing the panel; anouter edge facing away from the panel; an inner side facing the otherbatten; and an outer side facing away from the other batten.

Each batten has a batten depth between its inner and outer edges; and abatten width between its inner and outer sides. For each batten, thebatten depth may be greater than the batten width.

Optionally the panel assembly comprises a beam attached to the panel,the stiffener comprises a bridge which crosses over the beam at anintersection from a first side of the beam to a second side of the beam,the bridge has an outer surface facing away from the panel and an innersurface facing towards the panel, the inner surface of the bridgedeviates away from the panel to form a recess at the intersection, andthe outer surface of the bridge deviates away from the panel to form aprotrusion at the intersection.

Optionally the bridge comprises reinforcement fibres which extendcontinuously along the bridge and cross over the beam at theintersection from the first side of the beam to the second side of thebeam. Optionally at least some of said reinforcement fibres deviate awayfrom the panel at each end of the bridge.

A second aspect of the present invention provides an aircraft comprisinga stiffened panel assembly according to the second aspect.

A third aspect of the present invention provides a method ofmanufacturing a stringer according to the first aspect, the methodcomprising: assembling the core with the spacer between the battens;then surrounding the core with the shell.

A fourth aspect of the present invention provides a method ofmanufacturing a stiffened panel assembly according to the second aspect,the method comprising manufacturing the stiffener by a method accordingto the third aspect; then adhering the stiffener to the panel.

Optionally the stiffener is adhered to the panel by co-infusing thepanel and the fibre material of the shell with a matrix; then curing thematrix.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows an aircraft;

FIG. 2 shows a starboard wing of the aircraft;

FIG. 3 is a sectional view of a wingbox;

FIG. 4 is an isometric view of a stiffened panel assembly;

FIG. 5 is a plan view of the assembly of FIG. 4 ;

FIG. 6 is a side view of the assembly of FIG. 4 ;

FIG. 7 is a sectional view taken along line C-C in FIG. 6 ;

FIG. 8 is a sectional view taken along line B-B in FIG. 6 ;

FIG. 9 is a sectional view taken along line A-A in FIG. 5 ;

FIG. 10 is a sectional view taken along line D-D in FIG. 5 ;

FIG. 11A shows a first embodiment of the rib foot beam;

FIG. 11B shows a second embodiment of the rib foot beam; and

FIG. 12 is an isometric view of a stiffened panel assembly according toa further embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1 shows an aircraft 1 with port and starboard wings 2, 3. Each winghas a cantilevered structure with a length extending in a spanwisedirection 42 from a root to a tip, the root being joined to an aircraftfuselage 4. The wings 2, 3 are similar in construction so only thestarboard wing 3 will be described in detail with reference to FIGS. 2and 3 .

The main structural element of the wing 3 is a wing box formed by upperand lower cover panels 21, 22 and front and rear spars 6, 7 shown incross-section in FIG. 3 . The cover panels 21, 22 and spars 6, 7 areeach Carbon Fibre Reinforced Polymer (CFRP) laminate components. Eachcover panel has a curved aerodynamic surface (the upper surface of theupper cover panel 21 and the lower surface of the lower cover panel 22)over which air flows during flight of the aircraft. Each cover panelalso has an inner surface carrying a series of stiffeners 8 extending inthe spanwise direction 42. Each cover panel carries a large number ofstiffeners 8, only five of which are shown in FIG. 2 and only seven ofwhich are shown in FIG. 3 for purposes of clarity. A much larger numberof stiffeners may be applied across the chord of the wing. Eachstiffener 8 is joined to one cover panel but not the other. In the caseof an aircraft wing cover panel, the stiffeners 8 are commonly referredto as stringers, but the term “stiffeners” will be used below.

The wing box also has a plurality of transverse ribs, each rib beingjoined to the cover panels 21, 22 and the spars 6, 7. The ribs includean inner-most inboard rib 10 located at the root of the wing box, and anumber of further ribs spaced apart from the inner-most rib along thelength of the wing box. The wing box is divided into two fuel tanks: aninboard wing fuel tank bounded by the inboard rib 10, a mid-span rib 11,the cover panels 21, 22 and the spars 6, 7; and an outboard wing fueltank bounded by the mid-span rib 11, an outboard rib 12 at the tip ofthe wing box, the cover panels 21, 22 and the spars 6, 7.

The inboard rib 10 is an attachment rib which forms the root of the wingbox and is joined to a centre wing box 20 within the body of thefuselage 4. Baffle ribs 13 (shown in dashed lines) form internal baffleswithin the fuel tanks which divide the fuel tanks into bays. The ribs10, 11, 12 are sealed to prevent the flow of fuel out of the two fueltanks, but the baffle ribs 13 are not sealed so that fuel can flowacross them between the bays. As can be seen in FIG. 2 , the stiffeners8 stop short of the inboard rib 10 and the outboard rib 12, but passthrough the baffle ribs 13 and the mid-span rib 11.

Each rib 10, 11, 12, 13 connects the upper cover panel 21 to the lowercover panel 22, and FIG. 3 shows the upper and lower rib/coverconnection arrangements for the rib 11 by way of example. A rib footbeam 18 is adhered to the inner surface of each cover panel 21, 22, andattached to the rib 11 between the stiffeners 8 by fasteners 14 (such asbolts or rivets) which pass through the rib 11 and the rib foot beam 18.The stiffeners 8 pass through mouse-hole openings 20 in the rib 11.

Each stiffener 8 crosses over the rib foot beam 18 at an intersection.At each intersection the rib foot beam 18 is located between the panel21, 22 and a respective one of the stiffeners 8.

As noted above, the upper and lower cover panels 21, 22 provide theupper and lower walls respectively of a fuel tank. If the fuel tank isover-filled, then large fuel pressure forces can be generated which riskdetaching the rib foot beam 18 from the cover panel. The interlockingrib foot/stringer arrangement enables the stiffeners 8 to hold the ribfoot beam 18 down against the cover panel and prevent fuel pressureforces from separating the rib foot beam 18 from the cover panel.

The use of a single rib foot beam 18 per rib/cover interface makes theassembly less complex to manufacture. It is also not necessary to alignmultiple rib feet with each other.

FIGS. 4 and 5 show a panel assembly which includes the cover panel 22; arib foot beam 18; and stiffeners 8 carried on the inner surface of thecover panel 22.

FIG. 7 shows one of the stiffeners 8 in cross-section transverse to itslength. Each stiffener 8 comprises a core 30 and a shell 31. The shell31 has a closed cross-section and fully surrounds the core 30 on allsides. In this example the shell 31 has a substantially rectangularouter profile, with rounded corners, although other shapes are possible.

The shell 31 is formed from a fibre-reinforced composite material, suchas a carbon-fibre reinforced polymer. For example the shell 31 maycomprise a layer of woven fabric which is wrapped around the core 30, orit may be formed by braiding.

The shell 31 comprises a foot 32; a crown 33 opposite the foot 32; afirst side wall 34; and a second side wall 35 opposite the first sidewall 34. The shell 31 surrounds the core 30 on all sides.

The foot 32 of the shell is adhered to the cover panel 22. Beads ofadhesive 36 are applied where the rounded corners of the shell 31 meetthe cover panel 22.

Each side wall 34, 35 is longer than the foot 32, viewed in sectiontransverse to the lengthwise direction of the stiffener, as in FIG. 7 .Each side wall 34, 35 is also longer than the crown 33, viewed insection transverse to the lengthwise direction of the stiffener, as inFIG. 7 .

The first and second side walls 34, 35 are vertical and substantiallyparallel with each other. The stiffener 8 can be inspected by variousnon-destructive testing (NDT) techniques. In one example, ultrasound isdirected into the stiffener through one of its side walls 34, 35, andthe reflections analysed. The vertical orientation of the side walls 34,35 makes the stiffener easy to inspect in this way, because theultrasound is directed back to the NDT probe rather than being directedup at an angle by an oblique sidewall as in US2010129589. However, inother embodiments the shell 31 may have a trapezoidal section so thatthe first and second side walls 34, 35 are not parallel with each other.

The core 30 comprises first and second battens 40, 41; a spacer 50between the battens; and a cap 51 between the battens 40, 41 and thecrown 33 of the shell 31.

The battens 40, 41 are arranged side by side as shown in FIG. 7 . Eachbatten comprises an inner edge facing the panel 22; an outer edge whichfaces away from the panel 22 and is covered by the cap 51; an inner sidefacing the other batten; and an outer side facing away from the otherbatten.

Each batten has a shoulder where its outer edge meets its inner side.The battens are arranged shoulder to shoulder.

The first batten 40 is adjacent to the first side wall 34, and thesecond batten 41 is adjacent to the second side wall 35. In thisembodiment the outer sides of the battens are in contact with, andoptionally adhered to, the side walls 34, 35 of the shell. In otherembodiments the outer sides of the battens may be adjacent to the sidewalls 34, 35 of the shell without being in contact with the side walls34, 35 of the shell.

In this embodiment the battens 40, 41 have a rectangular cross-section,but this is not essential and other cross-sectional shapes are possible.

Preferably the core 30 has no more than two battens 40, 41. This makesNDT testing easy and simplifies the assembly of the core 30.

The battens 40, 41 are formed from a fibre-reinforced compositematerial, which may be a carbon-fibre reinforced polymer like the shell31, or another type of fibre-reinforced composite material.

The spacer 50 is formed from a material which is sufficiently rigid tocontrol the size of the gap between the battens 40, 41. For example thespacer 50 may be formed from a foam material.

The cap 51 is adjacent to, and in contact with, the crown 33 of theshell 31. The cap 51 is formed from a material which is sufficientlyrigid to provide impact protection. For example the cap 51 may be formedfrom a foam material, which may or may not be the same material as thespacer 50.

The battens 40, 41 and the shell 31 are structural components, with ahigher mass per unit volume than the spacer 50 and a higher mass perunit volume than the cap 51.

Each stiffener 8 extends in a lengthwise/spanwise direction indicated byan arrow 42 in FIGS. 2, 5, 6, 8 and 9 . The lengthwise/spanwisedirection 42 is the spanwise direction of the wing 3, extendingoutwardly towards the tip of the wing.

The battens 40, 41, the spacer 50 and the cap 51 have respective lengthswhich extend in the lengthwise/spanwise direction 42 of the stiffener 8as shown in FIGS. 8 and 9 . The battens 40, 41, the spacer 50 and thecap 51 extend continuously along a full length of the stiffener 8, fromits inboard end to its outboard end, or at least along a majority of afull length of the stiffener 8.

The battens 40, 41 are spaced apart across the width of the stiffener bya gap, and the spacer 50 fills the gap between the battens. The battens40, 41 are not in contact with each other at any point along theirrespective lengths.

The shell 31 has a depth (labelled D in FIG. 7 ) and a width transverseto the length of the stiffener (labelled Win FIG. 7 ). The battens 40,41 are spaced apart across the width of the shell 31, and the depth (D)of the shell is greater than the width (W) of the shell. In this examplethe aspect ratio (average shell depth/average shell width) is aboutfour, although it may vary. Typically, the aspect ratio of the shell isgreater than two, or greater than three.

The relatively high aspect ratio (depth/width), compared with thestiffener in US2010129589, makes the stiffener 8 lighter and easier toarrange on the panel with a small pitch between adjacent stiffeners.

The stiffener 8 also has a relatively low aspect ratio (depth/width),compared with a conventional blade stiffener, which makes it less proneto buckling and less prone to damage at its free edge.

Each batten 40, 41 has a batten depth between its inner and outer edges;and a batten width between its inner and outer edges. For each batten,the batten depth is greater than the batten width. In this example theaspect ratio (average batten depth/average batten width) is about ten,although it may vary. Typically, the aspect ratio of each batten isgreater than three, or greater than five.

Sandwiching a spacer 50 between the pair of battens 40, 41 enables thewidth of the stiffener 8 to be tailored by an appropriate selection ofthe width of the spacer 50.

Sandwiching a spacer 50 between the pair of battens 40, 41 also enablesthe mechanical properties of the stiffener 8 to be varied along thelength of the stiffener, by varying the relative widths of the spacerand the battens.

An example of this is shown in FIG. 8 . The battens 40, 41, the spacer50 and the gap between the battens 40, 41, have respective widths shownin FIG. 8 . The widths of the battens 40, 41 decrease in the lengthwisedirection 42, i.e. towards the tip of the wing, in a transition region52 shown in FIG. 8 . The widths of the spacer 50, and the gap betweenthe battens, increase in an opposite sense over the same transitionregion 52.

Thus the overall width of the stiffener—i.e. the width between the sidewalls 34, 35—does not change in the transition region 52. This ensuresthat the core 30 has a width and cross-sectional area which does notchange substantially along the length of the stiffener, even though thewidths and cross-sectional areas of the battens 40, 41 change.

The stringer 8 is manufactured by assembling the core 30 with the spacer50 between the battens 40, 41; then surrounding the core 30 with theshell 31, for instance by wrapping or braiding the shell 31 around thecore 30.

The stiffener 8 may be assembled as a dry-fibre preform, i.e. with theshell 31 and the battens 40, 41 formed from porous dry-fibre material.Alternatively, the stiffener 8 may be assembled as a prepreg, i.e. withthe shell 31 and the battens 40, 41 assembled from “prepreg”fibre-reinforced composite material.

The cover panel 22 may laid up on a mold tool as a dry-fibre preform,and the stiffeners 8 may be placed on the panel on the mold tool. Eachstiffener 8 may be assembled in prepreg and pre-cured before it isplaced on the cover panel 22, or it may be placed on the cover panel 22as a dry-fibre preform.

The cover panel preform on the mold tool is then infused with a matrixmaterial, which is then cured. The curing of the matrix material adheresthe stiffeners 8 to the panel 22. If each stiffener 8 is pre-curedbefore it is laid onto the panel 22, then the stiffener is adhered tothe panel 22 by a co-bonded joint. If each stiffener is placed on thecover panel 22 as a dry-fibre preform, then the stiffener and panel 22preforms are co-infused by the matrix material, so that each stiffener 8becomes adhered to the panel 22 by a co-cured joint.

The use of a shell 31 with a closed cross-section which fully surroundsthe core 30 is advantageous because it enables the stiffener 8 to beeasily assembled and handled “off-line” in an automated process, ratherthan being laid up “on-line” on a mold tool, as in US2010129589.

As shown in FIGS. 9 and 10 , each stiffener 8 comprises a respectivebridge 60 which crosses over the rib foot beam 18 at an intersectionfrom a first side of the rib foot beam 18 (on the left-hand side ofFIGS. 9 and 10 ) to a second side of the rib foot beam 18 (on theright-hand side of FIGS. 9 and 10 ).

Each bridge 60 has an outer surface 61 facing away from the panel 22 andan inner surface 62 facing towards the panel 22. The inner surface 62 ofeach bridge deviates away from the panel to form a recess 63 at theintersection, and the outer surface 61 of each bridge deviates away fromthe panel to form a protrusion 64 at the intersection.

Each protrusion 64 comprises a pair of ramps 70, and a flat apex 71between the ramps. Each ramp 70 is rounded where it meets the apex 71.Other shapes are possible: for instance the apex 71 and/or the ramps 70may be continuously rounded.

The inner surface 62 of each bridge follows a curved path 65 as itdeviates up and away from the panel. Other shapes are possible: forinstance the inner surface 62 may be continuously rounded.

As shown in FIG. 10 , each stiffener 8 comprises: a first stiffenerportion 66 which is attached to the panel 22 on the first side of therib foot beam 18, the first stiffener portion 66 having a firststiffener portion depth D1; and a second stiffener portion 67 which isattached to the panel on the second side of the rib foot beam 18, thesecond stiffener portion 67 having a second stiffener portion depth D2.The bridge 60 has a bridge depth D3 between its outer surface 61 and itsinner surface 62.

The bridge depth D3 at the apex of the protrusion 64 is substantiallythe same as the first and second stiffener portion depths D1, D2.

Each shell comprises a foot 32 shown in FIG. 7 which extendscontinuously across the bridge 60. As shown in FIG. 9 , the foot 32 hasa first foot portion 32 a which is adhered to the panel on the firstside of the rib foot beam 18, a second foot portion 32 b which isadhered to the panel on the second side of the rib foot beam 18, and abridge foot portion 32 c which deviates away from the panel at thebridge 60. An inner surface of the bridge foot portion 32 c provides theinner surface 62 of the bridge 60.

At each intersection a pair of bridge support structures 80, 81 areprovided. These comprise a first bridge support structure 80 between thebridge and the panel on the first side of the rib foot beam; and asecond bridge support structure 81 between the bridge and the panel onthe second side of the rib foot beam. The support structures 80, 81 arenot wrapped within the shell 31, and may be added as a part of the ribfoot beam 18 for the stiffener 8 to sit on. The support structures 80,81 may be made from a foam material or a carbon-fibre compositematerial.

Each foot 32 comprises reinforcement fibres which extend continuouslyalong the bridge 60 and cross over the beam at the intersection from thefirst side of the beam to the second side of the beam. Some or all ofthe reinforcement fibres in the foot 32 may deviate away from the panelat each end of the bridge 60. Some or all of the reinforcement fibres inthe rest of the shell 31 may also deviate away from the panel at eachend of the bridge 60.

Each batten 40, 41 comprises reinforcement fibres which extendcontinuously along the bridge 60 and cross over the beam at theintersection from the first side of the beam to the second side of thebeam. Some or all of the reinforcement fibres in the battens 40, 41 maydeviate away from the panel at each end of the bridge.

As shown in FIG. 11A, the rib foot beam 18 has a beam recess 90 at eachintersection. The beam recess 90 reduces the height of the rib foot beam18 at the intersection so that the bridge 60 does not have to deviatetoo far away from the panel.

Each beam recess 90 has a base 91 and a pair of angled side walls 92.The inner surface of each bridge 60 is seated in a respective one of thebeam recesses 90, in contact with the base 91.

The width of the rib foot beam 18 increases at each intersection, so thewidth of the rib foot beam 18 at the intersection (at the base 91 ofeach beam recess 90) is greater than the width of the rib foot beam 18at the protrusions 93 between the intersections.

FIG. 11B shows an alternative embodiment of the rib foot beam 18. Therib foot beam 18 in FIG. 11B is the same as the rib foot beam 18 in FIG.11A, except the base of each beam recess has a cut-out 95 with verticalside walls. The stiffeners 8 are received in the cut-outs 95.

Each bridge 60 has a pair of side walls 34, 35 connecting the outersurface 61 to the inner surface 62. In the embodiment of FIG. 11B, thevertical side walls of the cut-out 95 contact the side walls 34, 35 ofthe bridge 60. This provides support for the stiffeners 8, preventingthem from tipping over sideways.

Each stiffener 8 may be adhered to the panel 22 on each side of the ribfoot beam 18 by either a co-bonded or co-cured joint as described above.

Similarly the rib foot beam 18 may be adhered to the panel 22 by eithera co-bonded or co-cured joint.

Also, the inner surface 62 of each bridge may be adhered to the rib footbeam 18 by either a co-bonded or co-cured joint.

In summary, the aircraft wing 3 comprises an upper cover panel 21; alower cover panel 22; ribs 10, 11, 12, 13 connecting the upper coverpanel to the lower cover panel; and a plurality of stiffeners 8 attachedto the upper and lower cover panels. The ribs 11, 13 are joined to eachcover panel 21, 22 by a respective rib/cover connection arrangementshown in FIG. 3 . Each rib/cover connection arrangement comprises a ribfoot beam 18 which crosses the stiffeners 8 at a series of intersectionsand is attached to a respective one of the ribs 11, 13 between theintersections. Each stiffener 8 deviates away from the panel at eachintersection to form a respective protruding bridge 60 which crossesover the rib foot beam 18 at the intersection as shown in FIG. 4 . Thisimproves over the arrangement in WO2020/229501 since the bridge 60 canextend continuously across the intersection without any change in thecross-section of the stiffener.

Each stiffener 8 has two continuous load carrying components (battens40, 41) that run the length of the stiffener. These battens 40, 41 havea foam component 50 between them allowing any thickness increases in thebattens to be taken up inside the stiffener (foam thickness increasesand decrease) to keep the outer profile remaining the same. There is afoam cap 51 on top of the stiffener to stop edge impacts to thestructural plies and improve damage tolerance. Where the stiffeners 8interlock/cross the rib foot beams 18 there is a component (interlockingsupports 80, 81) to allow the structural components to sit on top. Thewhole stiffener 8 is then wrapped or braided.

FIG. 12 shows a stiffened panel assembly according to an alternativeembodiment of the invention. The lower cover panel 22 in FIG. 12 isidentical to the lower cover panel 22 in the previous embodiment.Stiffeners 108 are adhered to the panel 22. The stiffeners 108 areidentical to the stiffener 8, including all of the elements shown in thecross-section of FIG. 7 . The rib 11 is replaced by a rib 111 and therib foot beam 18 is omitted. The stiffeners 108 pass through mouse holesin the rib 111 without deviating to form bridges 60. The rib 111 has ribfeet 118 which are bolted to the panel 22 between the stiffeners 108.

The stiffened panels described above are covers for an aircraft wing,but the invention may be applied to other types of stiffened panelassembly for an aircraft. For example the stiffened panel assembly mayform a skin of an aircraft fuselage, the fuselage comprising longeronswhich extend in a fore-aft direction and frames which extendcircumferentially around the fuselage. In this case the longerons mayhave bridges which deviate from the skin and cross over the frames, orthe frames may have bridges which deviate from the skin and cross overthe longerons. Similarly the longerons may be formed as shown in FIG. 7, or the frames may be formed as shown in FIG. 7 . In the latter case,the lengthwise direction of the stiffener is the circumferentialdirection of the frame.

In other embodiments, the stiffened panel assembly may be part of adifferent vehicle, such as a boat or spacecraft; or it may be used insomething other than a vehicle.

Where the word ‘or’ appears this is to be construed to mean ‘and/or’such that items referred to are not necessarily mutually exclusive andmay be used in any appropriate combination.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

1. A stiffener comprising: a core; and a shell which surrounds the core,wherein the shell is formed from a fibre material, and the corecomprises: first and second battens arranged side by side; and a spacerbetween the battens, wherein the stiffener extends in a lengthwisedirection, and the battens and the spacer have respective lengths whichextend in the lengthwise direction of the stiffener.
 2. A stiffeneraccording to claim 1, wherein the battens are spaced apart by a gap; thespacer is in the gap between the battens; the battens, the spacer andthe gap have respective widths; the widths of the battens decrease inthe lengthwise direction; and the widths of the spacer and the gapincrease in the lengthwise direction.
 3. A stiffener according to claim2, wherein the core has a width which does not change substantiallyalong the length of the stiffener.
 4. A stiffener according to claim 1,wherein the battens are formed from a fibre material, which isoptionally a fibre-reinforced composite material.
 5. A stiffeneraccording to claim 1, wherein the fibre material of the shell is afibre-reinforced composite material.
 6. A stiffener according to claim1, wherein the battens have a higher mass per unit volume than thespacer.
 7. A stiffener according to claim 1, further comprising a capbetween the battens and the shell.
 8. A stiffener according to claim 1,wherein the shell has a depth and a width transverse to the length ofthe stiffener, the battens are spaced apart across the width of theshell, and the depth of the shell is greater than the width of theshell.
 9. A stiffener according to claim 1, wherein the shell comprises:a foot; a crown opposite the foot; a first side wall; and a second sidewall opposite the first side wall.
 10. A stiffener according to claim 9,wherein each sidewall is longer than the foot, viewed in sectiontransverse to the lengthwise direction of the stiffener.
 11. A stiffeneraccording to claim 9, wherein the first batten is adjacent to the firstside wall, and the second batten is adjacent to the second side wall.12. A stiffener according to claim 9, wherein the first and second sidewalls are substantially parallel with each other.
 13. A stiffeneraccording to claim 1, wherein each batten and each spacer extendscontinuously along a full length of the stiffener, or at least along amajority of a full length of the stiffener.
 14. A stiffened panelassembly comprising: a panel; and a stiffener according to claim 1adhered to the panel.
 15. A stiffened panel assembly according to claim14, wherein the shell comprises: a foot; a crown opposite the foot; afirst side wall; and a second side wall opposite the foot, wherein thefoot of the shell is adhered to the panel.
 16. A stiffened panelassembly according to claim 14, wherein each batten comprises an inneredge facing the panel; an outer edge facing away from the panel; aninner side facing the other batten; and an outer side facing away fromthe other batten.
 17. An aircraft comprising a stiffened panel assemblyaccording to claim
 14. 18. A method of manufacturing a stringeraccording to claim 1, the method comprising: assembling the core withthe spacer between the battens; then surrounding the core with theshell.
 19. A method of manufacturing a stiffened panel assemblyaccording to claim 14, the method comprising manufacturing the stiffenerby assembling the core with the spacer between the battens; surroundingthe core with the shell; and, then adhering the stiffener to the panel.20. A method according to claim 19 wherein the stiffener is adhered tothe panel by co-infusing the panel and the fibre material of the shellwith a matrix; then curing the matrix.